Complement promotes synaptic pruning by glia in order to refine neural circuits during development, but may also pathologically destroy mature, essential synapses in the context of neuroinflammation. Immune dysfunction and an imbalance in synaptic pruning have been implicated in autism spectrum disorder (ASD), and recent studies suggest that dysregulation of complement may be connected. The central hypothesis to be tested in this application is that the Sez6 gene family, whose members have been identified as ASD susceptibility genes, are novel, synaptically localized, complement regulators that are required to prevent aberrant synaptic pruning related to ASD. In Aim 1, we will define the complement regulatory properties of Sez6, Sez6L, and Sez6L2 using standard complement assays. We will then investigate whether the Sez6 family can prevent complement deposition at synapses in vitro. In Aim 2, we will use the 16p11.2 deletion mouse model of autism, in which Sez6L2 is lost along with other genes, to determine if enhanced complement dependent synapse pruning occurs at retinogeniculate synapses in the visual system. A Sez6L2 knockout will also be tested in order to validate findings and confirm the importance of SezL2 to the 16p11.2 phenotype. In aggregate, we expect the data obtained from these experiments to advance our understanding of the role of Sez6 proteins in mechanisms underlying complement-mediated synapse elimination during development that may explain how this gene family contributes susceptibility to ASD.